On Mon, 17 Oct 2011, Qu0ll wrote:
> I need to be able to encrypt/code an arbitrary string of up to about 50
> alphanumeric characters into a string that also contains only alphanumeric
> characters. All of the encryption algorithms I have looked result in strings
> with non-ASCII characters in them (when the resulting bytes are turned into a
> string) which is not suitable.
> Is there a simple way to do this? It must be able to successfully be
> decrypted as well but doesn't need to be very sophisticated or extremely
Oddly, i have a paper called "Ciphers with Arbitrary Finite Domains"
sitting in my reading queue right not.
You have at least two basic routes of attack here.
First, recognise that alphanumerism is just an encoding of a general bit
string. Decode the alphanumeric string into a bit string (by taking it as
a base-36 or base-62 number, or whatever), encrypt that, then re-encode
it. BigInteger has a constructor which takes a string and a radix, and a
toString method which takes a radix. So:
String s = "1sxjxyr5owpxwzmax6pyv1wgjpfuc4iadgrzhjpcameipq5sk";
BigInteger i = new BigInteger(s, 36);
i = i.multiply(BigInteger.valueOf(2)); // this is a very poor kind of encryption
BigInteger can also be converted to and from a byte, which you can
subject to proper encryption. You will need to be a bit careful, because
conversion to an alphanumeric string will remove any leading zeroes, so
you may need to pad. Also, the numbers may be negative, in which case the
alphanumeric strings will have a leading minus sign. You might prefer to
write your own conversion between bytes and digits, to avoid these
Note that using a proper cipher involves generating an initialisation
vector (IV) for each message you encrypt, which you will then need to send
along with the ciphertext. That's going to be slightly annoying, since the
alphanumerically encoded IV is likely to be just as long as your message.
Second, come up with a cipher that works directly on alphanumeric values,
rather than bit strings, and apply that to your string. I don't think this
is actually a terribly good idea, so i won't elaborate on it.
Per Dementia ad Astra